Substrate limitation responsive biosensors have been used for the development of a mini-bioreactor platform that can be used as a scale-down tool. Three green fluorescent protein (GFP) transcriptional ... [more ▼]

Substrate limitation responsive biosensors have been used for the development of a mini-bioreactor platform that can be used as a scale-down tool. Three green fluorescent protein (GFP) transcriptional reporters have been chosen in Escherichia coli, i.e., uspA::gfp, csiE::gfp and yciG::gfp. Our previous studies have shown that these kinds of promoters are induced in response to substrate limitation and are significantly repressed when cultures are carried out in heterogeneous bioreactors. This sensitivity to substrate limitation has been confirmed in the case of the csiE and yciG biosensors. A mini-scale-down platform is proposed as a high throughput tool to rapidly investigate the usefulness of a given microbial biosensor. This platform is composed of shake flasks able to operate in fed-batch mode either using the slow release or the intermittent feeding principle. Local heterogeneities were reproduced at the level of these mini-bioreactors (operating under the intermittent feeding principle) and caused a decrease in GFP expression as in conventional scale-down reactors. The presence of GFP in supernatants was also noted and seems to be correlated with the substrate limitation signal for the three cultivation systems considered in this work (i.e., chemostat, conventional and mini-bioreactors) and with membrane permeability. [less ▲]

The basic principle adopted in our studies is to use substrate limitation responsive biosensors in order to detect spatial glucose heterogeneities inside industrial bioreactors (whole-cell biosensor). Indeed, such heterogeneities cause a lowering of the biomass yield and an increase of by-products concentration. In our previous works, green fluorescent protein reporters have been used as biosensors of the heterogeneities generated in a two compartment scale-down reactor. As there is a huge variety of available whole cell biosensor to characterize the impact of such heterogeneities at the biological level, there is a need for high-throughput cultivation tools in order to investigate the usefulness of a given microbial biosensor among a library comprising several thousands of clones. This work is based on this statement and aims to investigate the potentialities of a mini scale-down platform. Four green fluorescent protein (GFP) transcriptional reporters have been chosen in Escherichia coli: rpoS::gfp, uspA::gfp, csiE::gfp and yciG::gfp. The promoters rpoS and uspA are induced in response to a variety of stresses whereas the two other promoters, csiE and yciG, are supposed to be more specific in front of a glucose limitation. First, the response of these biosensors has been assessed in chemostat reactors. These kinds of experiments allow easier interpretation of responses of stress gene related to a glucose limitation since the extracellular conditions are constants and cells are renewed. Biosensors carrying the csiE and yciG promoters have exhibited an induction in function of the glucose limitation. Secondly, a scale-down platform has been tested with the same biosensors and two kinds of glucose addition mode. This scale-down platform involves high-throughput cultivation tools, i.e. in our case shake flask, equipped with non-invasive optical sensors for the monitoring of the dissolved oxygen profile in front of the glucose addition mode. The first system is based on a commercial package (Enbase) based on the enzymatic release of glucose in the medium. The Enbase system allows the generation of a very smooth glucose profile without any perturbations. For comparison purpose, we have also used an intermittent feeding that induces strong fluctuation at the level of the glucose and the dissolved oxygen concentration. The intermittent addition of glucose induces a slow down at the level of the GFP synthesis, suggesting that temporal accumulation of glucose inhibits the activity of the yciG and csiE promoters. In conclusion, the scale-down platform is able to reproduce the same kind of glucose fluctuations that encounters the cells in large-scale processes but not allows studying the impact of high-cell density culture on gene expression. [less ▲]

The basic principle adopted in our studies is to use substrate limitation responsive biosensors in order to detect spatial glucose heterogeneities inside industrial bioreactors (whole-cell biosensor). Indeed, such heterogeneities cause a lowering of the biomass yield and an increase of by-products concentration. In this work, we have used these biosensors for the elaboration of a mini-bioreactor platform that can be used as a scale-down tool. Three green fluorescent protein (GFP) transcriptional reporters have been chosen in Escherichia coli, i.e. uspA::gfp, csiE::gfp and yciG::gfp. Our previous studies have shown that these kinds of promoters are induced in response of substrate limitation and exhibit a strong fluorescence attenuation when cultivated in heterogeneous bioreactors. This sensitivity to substrate limitation has been confirmed in the case of the csiE and yciG biosensors. A mini scale-down platform has been proposed as a high throughput tool to investigate rapidly the usefulness of a given microbial biosensor. This platform is composed of shake flask able to operate in fed-batch mode either by using the slow release or the intermittent feeding principle. The first system is based on a commercial package (Enbase) based on the enzymatic release of glucose in the medium. The Enbase system allows the generation of a very smooth glucose profile without any perturbations. For comparison purpose, we have also used an intermittent feeding that induces strong fluctuation at the level of the glucose and the dissolved oxygen concentration. Local heterogeneities have thus been reproduced at the level of these mini-bioreactors and these one have caused a decrease of GFP expression, as in conventional scale-down reactor. The presence of GFP in supernatants has also been noticed and seems to be correlated with the substrate limitation signal for the three cultivation systems considered in this work (i.e., chemostat, conventional and mini-bioreactors) and with the membrane permeability. [less ▲]

Substrate limitation responsive biosensors have been used in order to detect spatial substrate heterogeneities, , inside industrial bioreactors (whole-cell biosensor). Three green fluorescent protein (GFP) transcriptional reporters have been chosen in E.coli, i.e. uspA::gfp, csiE::gfp and yciG::gfp. The promoter uspA is induced in response to a variety of stresses whereas the two other promoters, csiE and yciG, are supposed to be more specific in front of a substrate limitation. The responsiveness of these biosensors has been assessed in chemostat reactor. Secondly, the same biosensors have been tested in well-mixed laboratory reactors and in scale-down reactors able to reproduce industrial conditions. Finally, a mini scale-down platform has been proposed as a high throughput tool to investigate rapidly the usefulness of a given microbial biosensor. Local heterogeneities in mini-bioreactor have caused a decrease of GFP expression, as in scale-down reactor. The presence of GFP in supernatants was noticed and this leakage seems to be correlated with the membrane permeability. [less ▲]

The dynamics of microbial stress response in intensive cultivation conditions remains not completely understood. In this work, two green fluorescent protein (GFP) transcriptional reporters have been used ... [more ▼]

The dynamics of microbial stress response in intensive cultivation conditions remains not completely understood. In this work, two green fluorescent protein (GFP) transcriptional reporters have been used as biosensors of the heterogeneities generated in a two-compartment scale-down reactor. The stress promoters have been chosen for their responsiveness to carbon limitation corresponding to the global substrate profiles encountered in intensive fed-batch cultures. From our results, it can be concluded that the exposure of microbial cells to substrates heterogeneities tends to decrease the GFP expression level in fed-batch mode. Fluorescence intensities have been monitored at the single cell level by using flow cytometry. During the course of the fed-batch culture, a drop at the level of the intracellular GFP content has been observed for the two scale-down operating conditions and for the two promoters sensitive to substrate limitation (rpoS and csiE). The fluorescence drop can be attributed to the repression of these promoters but also to the release of GFP to the extracellular medium according to the increase of the fluorescence level of the supernatant. This leakage has been observed for all the operating conditions, i.e. the scale-down reactors and the culture operating in the normal mode, i.e. in a well-mixed bioreactor. Interestingly, GFP leakage is more pronounced in the case of the cultures operated in the normal mode. Indeed, staining by propidium iodide tends to be more elevated for the microbial cells cultured under the normal mode by comparison with those cultured in scale-down conditions, indicating a higher permeability of the membrane. These results suggest that GFP microbial biosensors could be used to detect simultaneously mixing imperfections and their impact on the viability of microorganisms. [less ▲]